Polyurethane has high elasticity and flexibility, and is excellent in abrasion resistance and the like. Thus, it is used in many fields such as foams, elastomers, adhesives, and coatings. Polyurethane is synthesized mainly from polyisocyanate and polyol. As a polyol, polypropylene glycol (PPG) or an ethylene oxide-modified form thereof is most frequently used. The reasons for this are not only that these substances are inexpensive, but also that they can be handled very easily due to their liquid form having low viscosity at room temperature.
However, PPG does not have sufficient thermal resistance, weather resistance, mechanical characteristics, and the like. Thus, when these characteristics are required, a polyester polyol is used. Examples of a polyester polyol include: adipate-based polyester polyols, such as polyester polyol synthesized from ethylene glycol and adipic acid, and polyester polyol synthesized from 1,4-butylene glycol and adipic acid; and polycaprolactone-based polyester polyol obtained by using ε-caprolactone as a raw material (Japanese Laid-Open Patent Publication No. 2003-3057, for example).
Furthermore, polyester polyol is used not only as a raw material for polyurethane but also as a raw material for plasticizers, resin modifiers, and polyester or epoxy resin coatings (Japanese Laid-Open Patent Publication No. 10-265656, for example).
However, in general, a polyester polyol having a molecular weight of 1000 or more is a wax-like solid at room temperature because its melting point is 40 to 60° C. Thus, it is difficult to use such a polyester polyol in the case where a liquid form is required. Furthermore, the viscosity of the molten state is high, and, thus, working properties is extremely poor.
Disclosed examples of biodegradable polyester polyol include: lactic acid-modified polyol obtained by ring-opening copolymerization of lactides and alkylene oxide having 2 to 16 carbon atoms to a compound having two or more active hydrogen atoms in its molecule; and lactic acid-modified polyol obtained by addition polymerization of an alkylene oxide having two or more carbon atoms to a polylactic acid having a number average molecular weight of 100 to 2500 (Japanese Laid-Open Patent Publication No. 11-43538). Polyurethane or epoxy resin obtained from this polyester polyol exerts excellent biodegradability without impairing mechanical strength.
Polylactic acid obtained by polymerizing lactide or lactic acid is polyester made from renewable resources. However, due to its high crystallinity and high melting point, this polylactic acid has a problem in that the working properties is poor when used in various applications such as a raw material for resins and coatings.